OMAPDSS: DISPC: fix context restore

DISPC_MSTANDBY_CTRL register is used in the driver, but it's not
restored in dispc_restore_context(), causing problems after resume.

Instead of adding DISPC_MSTANDBY_CTRL to dispc_restore_context(), let's
call _omap_dispc_initial_config() as the first thing in
dispc_runtime_resume(). This will initialize the DISPC core registers
properly, and will avoid similar issues in the future.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>